The present invention relates to a process for the extraction of fission products, particularly radioactive iodine, cesium, rubidium and tritium contained in irradiated nuclear fuel elements and more particularly a process permitting the isolation, followed by the recovery of all the radioactive iodine during the initial wet processing operations or irradiated nuclear fuels and more particularly prior to dissolving the fuel of the latter.
Following irradiation nuclear fuel elements contain various fission products which must be isolated and stored because of their potentially harmful activity.
Among these fission products iodine constitutes one of the most harmful elements since it exists in the form of iodine 129 and iodine 131 which are beta gamma emitters. Iodine 129 has the additional disadvantage of being stable on the human scale because its half-life is 17.2.times.10.sup.6 years.
Moreover, in irradiated fuel treatment processes utilising a preliminary mechanical decanning or cutting of the fuel followed by dissolving this fuel in a nitric medium, numerous problems are experienced in the recovery of all the radioactive iodine because it escapes from the irradiated fuel in the initial treatment operations. The radioactive iodine is firstly off in the form of traces in the gases of the mechanical decanning or cutting installation for the irradiated fuel, then in very large quantities during nitric dissolving, either in gaseous form or in soluble form in the nitric solution.
Hitherto the extraction and recovery of radioactive iodine have mainly been carried out by treating the gaseous effluents. Such treatments mainly comprise washing the gaseous effluents with an appropriate solution, but they have the disadvantage of leading to large volumes of effluents from which it is often difficult to bring the iodine into a stable and concentrated form with a view to its storage.
Another iodine extraction method has been developed according to which the nitric acid recombined before its recycling is treated in the irradiated fuel dissolving stage in order to desorb most of the iodine contained therein.
It has also been envisaged to treat gaseous effluents by means of silver-charged solid absorbents which permit the obtaining of high retention factors but whose industrial utilisation is only economically practicable for a final trapping for the purpose of holding back the final traces of iodine.
The above treatment methods have the important disadvantage of being unable to ensure the extraction and recovery of all the radioactive iodine present in the irradiated fuels, because they do not make it possible to prevent a certain quantity of iodine from remaining in the nitric dissolving solution when the oxidoreduction conditions prevent the passage of the iodine in the volatile elementary state or when the entrainment conditions of the volatile iodine have not been combined. As a result, this quantity of iodine is then disseminated in the following treatment phases of the irradiated fuels and is finally discharged in an anarchic manner in the plant effluents.
In addition, such methods do not simultaneously ensure an extraction of the radioactive cesium, rubidium and tritium which are also present in the irradiated fuels.